1. Field of the Invention
The present invention relates generally to a process producing oriented electrical steel sheet having a high magnetic flux density. More particularly, the present invention relates to a process of producing oriented electrical steel sheet of the type wherein most of crystal grains is aligned with each other with a certain specific orientation such as (110) &lt;001&gt;, (100) &lt;001&gt; or the like represented by a mirror index.
The steel sheet produced by employing the process of the present invention is used as soft magnetic material for producing cores for various kinds of electric apparatuses, electric equipment or the like.
2. Description of the Prior Art
Oriented electrical steel sheet has a structure composed of crystal grains aligned with each other with a specific orientation as mentioned above wherein each steel sheet usually contains Si of 4.8% or less and has a thickness ranging from 0.10 to 0.35 mm. These steel sheet is required to have excellent magnetizing properties and iron loss properties as magnetic properties. To satisfactorily meet the requirement, it is important that crystal grains are aligned with each other with an exact orientation. Integral alignment of the crystal grains with each other with the specific crystal orientation has been accomplished by utilizing a phenomenon of catastrophic grain growth called secondary recrystallization.
To properly control the secondary recrystallization, it is necessary that a primarily recrystallized structure is properly adjusted prior to the secondary recrystallization, and moreover, a fine precipitated substance called an inhibitor or a grain boundary segregated type element is properly adjusted prior to the secondary recrystallization. The inhibitor has a function of suppressing the growth of general primarily recrystallized grains in a primarily recrystallized structure to thereby selectively grow crystal grains each having a certain specific orientation.
According to the reports given by M. F. Littman (official gazette of Japanese Examined Publication Patent (Kokoku) No. 30-3651) and J. E. May and D. Turnbull (Trans. Met. Soc. AIME 212 (1958) P769/781), MnS is noted as a typical precipitated substance. In addition, according to the report given by Taguchi and Sakakura (official gazette of Japanese Examined Publication Patent (Kokoku) No. 40-15644), AlN is noted as a typical precipitated substance. Additionally, according to the report given by Imai et al. (official gazette of Japanese Examined Publication Patent (Kokoku) No. 51-13469), MnSe is noted as a typical precipitated substance. Further, Komatsu et al. reported that (Al, Si)N is a typical precipitated substance.
On the other hand, according to the report given by Saito (Trans. of Japanese Metal Association, Vol. 27 (1963) P186/195), Pb, Sb, Nb, Ag, Te, Se and S are noted as grain boundary segregated type elements but they are used practically merely as an assistant for the inhibitor on an industrial basis.
At present, it is not necessarily clarified what the necessary conditions are for allowing each of the above-noted precipitated substances to function as an inhibitor but it is considered based on the results obtained from the reports given by Matsuoka (Iron & Steel, Vol. 53 (1967) P1007/1023) and Kuroki et al. (Trans. of Japan Metal Association, Vol. 43 (1979) P175/181) and (Trans. of the same, Vol. 44 (1980) P419/427) that the following conditions are necessary for the same purpose as mentioned above.
(1) A sufficient quantity of fine precipitated substance enough to suppress growth of primarily recrystallized grains is present prior to secondary recrystallization.
(2) Each precipitated substance has a considerably large size, and moreover, it is not thermally transformed at an excessively high rate during a secondary recrystallization annealing operation.
At present, the following three kinds of methods can each be noted as typical methods for producing grain oriented electrical steel sheets on an industrial basis.
Specifically, a first prior technology is disclosed in an official gazette of Japanese Examined Publication Patent (Kokoku) No. 30-3651 of M. F. Littmann wherein MnS is used as a precipitated substance to enable a hot rolled sheet to be subjected to cold rolling twice, a second prior technology is disclosed in an official gazette of Japanese Examined Publication Patent (Kokoku) No. 40-15644 of Taguchi and Sakakura wherein AlN+MnS are used as precipitated substances to enable a cold rolled sheet to be subjected to final cold rolling at a high reduction ratio exceeding 80%, and a third prior technology is disclosed in an official gazette of Japanese Examined Publication Patent (Kokoku) No. 51-13469 of Imanaka et al. wherein MnS (or MnSe)+Sb are used as precipitated substances so as to enable a hot rolled sheet to be subjected to cold rolling twice.
To satisfactorily meet the requirements for assuring a quantity of precipitated substance and minimizing it in size, each of the aforementioned prior technologies is practiced based on the fundamental technical concept that an inhibitor is prepared by heating a slab of silicon steel up to an elevated temperature exceeding 1270.degree. C. prior to a hot rolling operation.
However, when the slab is heated to an elevated temperature as mentioned above, the following problems occur.
1) It is necessary that a high temperature slab heating furnace exclusively employable for producing oriented electrical steel sheets be installed in a steel plant.
2) The energy unit cost required for operating the slab heating furnace is high.
3) The surface of each slab is promotively oxidized and a molten material called slag appears, resulting in adverse operation of the slab heating furnace.
To obviate the above problems, there has arisen a necessity for developing a technology for preparing an inhibitor without the need of heating a slab to an unusually high temperature.
Some of the inventors have proposed a method of preparing an inhibitor by performing a nitriding operation, as disclosed in an official gazette of Japanese Examined Publication Patent No. (Kokoku) No. 62-45285 (grain oriented electrical steel sheet) and official gazette of Japanese Unexamined Publication Patent (Kokai) No. 1-139722 (double oriented electrical steel).
A significant feature of the proposed process is that inhibitors are uniformly precipitated and dispersed in the steel sheet. However, when the process is practiced on an industrial scale, it was found that if the nitriding operation is irregularly performed in the longitudinal direction of a coil or in the transverse direction of the same, magnetic properties of the steel sheet become correspondingly irregular.
In view of the aforementioned problem, a proposal has been made regarding a method of nitriding a steel sheet (strip) using a gas such as an ammonia gas or the like having a nitriding function, as disclosed in an official gazette of Japanese Unexamined Publication Patent (Kokai) No. 1-91956. This prior invention makes it possible to uniformly nitride a steel sheet in the longitudinal direction of a coil and also in the transverse direction of the same.
However, even when a steel sheet is uniformly nitrided in the longitudinal direction of a coil and in the transverse direction of the same, a satisfactory secondary recrystallizing operation does not necessarily occur for reasons that have not been made clear in the past.
The present invention has been made with the above background in mind and its object resides in clarifying the essential reason why a satisfactory secondary recrystallizing operation does not in some cases occur. Another object of the present invention is to present operational conditions for assuring that a satisfactory and stable secondary recrystallizing operation does occur.